According to the passage, what did small children do when they were asked to count a pile of red and blue pencils a. They counted the number of pencils of each color. b. They guessed at the total number of pencils. c. They counted only the pencils of their favorite color. d. They subtracted the number of red pencils from the number of blue pencils.

寻呼系统经常对民航频率造成干扰的原因之一是寻呼频率的互调产物正好落入民航频段中。